The invention relates to the field of data collection and measurement systems, and particularly to methods of calibrating these systems.
Systems that make and use measurements of light, weight, energy, mass, volume and the like are pervasive in modem society, from the simplest instruments such as a scale, to very complex instruments from radio telescopes to medical imaging systems. Regardless of the measurement system, ultimately all are susceptible to fluctuations in operating and environmental conditions that affect the accuracy of the measures being made. For example, everyone has encountered the need to calibrate or zero a scale because it produces a reading other than zero when nothing is on the scale. These errors in measure may be the result of environmental conditions such as temperature, humidity, magnetic fields, or physical condition such as wear or debris on the scale. The process of calibration or zeroing in this simple case is to readjust the measure to account for the inaccuracy, in effect subtracting or adding to the measure to zero the measure.
More complex measuring systems present more complex and difficult problems for calibration. For example, one category of complex measurement systems are those found in medical instrumentation that employ multiple detectors to measure properties of the human body, such as electrocardiograph systems, electroencephalograph systems, optical tomography systems, magnetic resonance imaging systems and the like.
Exemplary multi-channel measuring systems can be found in the field of optical tomography. Optical tomography systems, such as that shown schematically in FIG. 1, have source channels and detector channels arranged around a target medium. The source channels direct energy into the target at a location and the detector channels measure the scattered energy emerging from the target. Based on these multi-channel measures of energy, a cross-sectional image of the target medium is generated.
In these systems the images to be reconstructed from the detector measurements are functions of the combined measures from all the detectors. Accordingly, it is critical that the detectors be calibrated so that the measures between detectors have a quantifiable relationship, that is, so that the measure from one detector has meaning in relation to another detector. A problem with multi-channel systems is that efficiencies are likely to vary from channel to channel of the measurement system, so that the measurement from one channel may not have a quantifiable relationship to the measure from another channel. The varying efficiencies from channel to channel are a function of coupling losses between the energy source and the source fiber, coupling losses at the interface between the target and source and detector fibers, source and detector fiber transmission losses, and coupling losses at the interface between the detector fiber and detector.
One known means for calibrating an optical tomography system is to directly measure the source and detector channel efficiencies individually. For example, if each source channel has an energy source and a fiber for delivering the energy, the loss through the channel can be ascertained by using a power meter to measure the energy exiting the end of the fiber. Assuming the energy originating from the source is known, the loss is the known source energy emitted from the source minus the measured energy existing at the end of the fiber. This process can be repeated for each source channel. Similarly, if each detector channel comprises a detector fiber and a detector, the loss through the detector channel can be ascertained by directing a known energy source into each detector fiber and measuring the energy exiting the fiber at the detector. Knowing the energy of the source, the loss in the channel is the known source energy minus the measured energy.
Although this and other approaches to calibration have been adopted for multi-channel measurement systems, none of these known approaches (1) permit calibration of the system, fully assembled, with all components disposed as they would be during an actual measurement, or without the introduction of components external to the system as it is used in an actual measure, or (2) account for both the varying systematic performance among the measuring channels while also providing statistical measures that facilitate system troubleshooting in the case of component (system) degradation or failure.
The ability to calibrate the measuring system with all components disposed as they would be during an actual measurement is especially important for practical clinical use because the system losses and corresponding calibration values are susceptible to variations in the position of system components. For instance, the fiber transmission losses will vary as a function of the bending and positioning of the fiber, and fiber-target coupling losses are a function of the interface between the fiber and the target medium.
In addition, the ability to derive statistical data associated with the estimates of channel efficiencies would not only permit improved estimation of the channel efficiencies, but also provide insight on system performance that is useful for system troubleshooting.
For the foregoing reasons, there is a need for (1) a multi-channel calibration protocol wherein the calibration measurements can be made with the instrument fully assembled, all components disposed as they would be during an actual measurement and without introducing source or detector components in addition to those used in an actual measurement, and (2) a means of providing statistical measures of the component efficiencies.
The present system and method satisfies these needs by (1) providing a calibration protocol wherein the measurement for calibration are made with the instrument fully assembled and all components disposed as they would be during an actual measurement, and (2) providing a plurality of efficiency estimates for each channel whereby providing statistical measures can be generated.
It is an object of the present invention to provide a system and method for calibrating a multi-channel measurement system wherein the measurement system is calibrated by directing energy through a source channel into a calibrating target medium at a plurality of source locations around the target. For each source location, energy emerging from the calibrating target medium is measured at a plurality of locations around the target using a plurality of detector channels. The measured energy for the plurality of detector channels for each source location is then processed using an iterative proportional fitting technique to determine a relative value of energy loss in the calibrating target medium. The relative energy loss or efficiency of each source and/or detector channel is then determined based on the measured energy in each detector channel and the relative values of energy loss in the target medium. The relative energy loss for each channel is then used to establish the gain required to calibrate the channels. Thereafter, measurements of an actual target medium are taken and adjusted based on the gain calculation. on the measured energy in each detector channel and the-relative values of energy loss in the target medium.
Further features, aspects and advantages of the invention will be apparent from the following detailed description of the preferred embodiment and accompanying drawings.